1. Field of the Invention
The present invention relates to a refrigerating apparatus provided with a fixed displacement type compressor for refrigeration, and, more particularly, to a method and apparatus for preventing an overheating of a compressor over a wide pressure range by introducing a part of a high pressure liquefied coolant, condensed in a condensor of a refrigeration cycle, into a compression chamber kept under a compression stroke of the compressor through a communicating pipe.
2. Description of the Prior Art
A conventional method for preventing an overheating of a compressor by introducing a high pressure liquefied coolant, condensed in a condensor, into a compression chamber of the compressor has been applied to many types of compressors, and a high pressure liquefied coolant has been introduced into the compression chamber of the compressor in order to prevent an overheating of the compressor. For example, Japanese Patent Unexamined Publication 60-166778, proposes a displacement compressor in which overheating is prevented by introducing a high pressure liquefied coolant into a compression chamber of the compressor.
In a fixed displacement type compressor, an average pressure at a position where a communicating pipe is in fluid communication with a compression chamber maintained under the compression stroke of the compressor is determined substantially in dependence upon the operational pressure of a lower pressure side thereof and the position at which the communicating pipe is connected to the compression chamber. The introduction of the liquefied coolant is performed in accordance with a pressure differential between the operational high pressure liquefied coolant pressure and the average pressure at the communicating position of the communicating pipe derived from the compression chamber under the compression stroke within the compressor, only when the former pressure is higher than the latter pressure. Under such a specific operational condition, it would be impossible to introduce the coolant into the compression chamber because the former pressure is lower than the latter pressure and consequently the compressor would overheat. Also when the latter pressure is extremely low relative to the former pressure and the pressure differential therebetween is increased, the amount of the introduced liquefied coolant is increased. As a result, the consumption of electric power increases due to the increase of the gas compression power or the compressor would be excessively cooled down. There has been no consideration given to the communicating position of the communicating pipe for introducing the liquefied coolant or of the cooling condition and compression power of the compressor. In particular, in the case where the compressor is used over a wide operational pressure range, there is a problem that, depending upon the introduction position of the liquefied coolant, the cooling effect would be insufficient at the low operational pressure ratio, the cooling efect would be excessive at the high operational pressure ratio, and the consumption electric power due to the increased compression power would be increased. In a compressor which may be cooled over the wide operational pressure range by introducing the liquefied coolant through one position, there would be a problem that an undesired compression power would be increased in accordance with the introduction of the liqufied coolant in particular on the lower evaporation temperature side (high operational pressure ratio).
With respect to an apparatus for preventing an overheating of the compressor by introducing a high pressure liquefied coolant into the compression chamber under the compression stroke within the compressor in a refrigeration cycle formed by the prescribed displacement type compressor, no consideration has been given to the communicating position of the connecting pipe for introducing the high pressure liquefied coolant during the compression stroke. In the fixed displacement type compressor, in the compression stroke, the compression is forcibly performed up to a prescribed pressure ratio determined in dependence upon the prescribed volume ratio and the coolant to be used, and thereafter, the condensation pressure is obtained by communicating the coolant to the discharge space. Accordingly, there are cases where the condensation pressure is lower than the average pressure of the communicating position of the connecting pipe during the compression stroke. In these cases, it is impossible to introduce the liquefied coolant therein. When the compressor is overheated or the evaporation pressure is low, the pressure differential between the communicating position pressure of the connecting pipe and the condensation pressure is increased so that the amount of the liquefied coolant introduced is increased and the compressor is excessively cooled. As a result, the consumption of electric power is increased because of the increase of the compression power, or the compression mechanism is damaged due to the liquid compression. At this time, even if the introduction of the liquefied coolant is controlled by the opening/closing operation of the solenoid valve, the opening/closing operations are frequently performed so that a stable operational condition of the compressor can not be attained.
On the other hand, when the system is used over a wide operational pressure range, in order to perform a suitable cooling effect at a high evaporation temperature, it is necessary to set the communicating position of the connecting pipe at a lower pressure side. Accordingly, this system suffers from problems such as excessive cooling and an increase of the compression power on the lower evaporation temperature side.